Hi hawtreee.
Welcome to the Norton Community Forums. 
You are right to say such things have been seen here before but you seem to have it worse than I can recall having seen it. Congratulations! 
N360 is supposed to do an incremental backup after its first full backup. This should mean that the backup file size would grow slowly. However for some reason unknown to me it sometimes seems to do a complete new backup and put it on top of the old backup which then becomes inaccessible (just the old one not the new one). However the re-run of the full backup has seemed to occur only occasionally and for reasons I do not know.
The rapidity of your backup file growth may provide us with an opportunity to study this issue and get to the bottom of it. However you may just wish to get rid of the problem and move on.
If you are happy to try doing some research then let me know and I will try to contact the relevant people on the team and help you to collect some data which may help. However if you just want to get things clear I am happy to try and do that also. Just say what you want to do.
It would help to know how large N360 thinks the backup size should be (Backup - Manage Backup Sets - Preview) and when you have set up the backup to run (is it on Automatic or what?). Is the connection to drive D: wired or wireless (I assume wired)?
Also have you tried "Support - Get Support" from the main N360 screen and if so did it report finding anything odd?
I look forward to hearing back.
All the best.